Method for the preparation of hexafluorobenzene



NIETHOD FOR THE PREPARATION OF HEXAFLUOROBENZENE '5 Claims. c1. ido-650) a This invention relates to animproved methodfor the production ofhexafluorobenzene and particularly contemplates the production of suchcompound from the pyrolysis of tribromofiuoromethane. I

In accordance with the principles of, this inventiontribromofluoromethane is pyrolyzed in graphite-platinum, and nickeltubesat temperatures. ranging from 500 to 750 C. The resulting yield ofhexafluorobenzene is superior compared to known methods of preparation.

Hexafluorobenzene is becoming increasingly important as a precursor forthe preparation of thermally stable liquids and polymers and accordinglyconsiderable research is being directed to the preparation of suchcompound. Previously hexafluorobenzene was synthesized by thefluorination of hexachlorobenzene with bromine trifluoride followed bytreatment of the intermediate with antimony pentafiuoride and finally bydehalogenation with the zinc. Relatively small amounts ofhexafluorobenzene were obtained in accordance with such process. I Amore direct method gives higher yields was described by Y. Desirant inthe Bulletin of the Academy of the Royal Belgian Classe Sci, Inaccordance with such described process tribromofluoromethane waspyrolyzed in a platinum tubeat temperatures ranging from 630640 C. Thereported yield of hexafluorobenzene was 45 percent on the basis of thereactant tribromofiuoromethane. Our yields under these conditions wereabout 30 percent.

The present invention provides a convenient method for synthesizinglarger amounts of hexafluorobenzene by the pyrolysis oftribromofluoromethane .by varying the temperature and pressureconditions of reaction.

It isaccordingly an immediate object of the'present inventionto providean improved method 'for 'the'preparation of hexafluorobenzene. f

nited States meato "ice The pyrolysis reaction can be represented by thestoichiometric equation p a small amount of bromine.

, cocoanut charcoal was used as packing.

RESULTS Table I summarizes the results of all runs performed ingraphite, Vycor, and platinum tubes. The data pre- 1 sented are yieldsof hexafluorobenzene (gross and net,

In accordance with such objective it is the purpose of this invention toprovide relatively large yields of hexafluorobenzene from the pyrolysisof tribromofluoromethans;

suresabove atmospheric toabout 20 atmospheres. I

' It is a still further object of the present invention to columns 7 and9), the recovery of unreacted tribromofluoromethane (column 8), and theamount of bromine liberated in the pyrolysis (column 6). In the graphitetubes with charcoal packing the best yield of hexafluorobenzene wasobtained at the lowest temperature, about 650. This yieldwas 24 percentor 30 percent based on reacted tribromofluoromethane. Increasingtemperatures lowered the yield of hexafiuorobenzene, although the amountof bromine liberated was increased. The two runs performed with platinumgauze as packing in graphite tubes showed a reverse trend inasmuch asthe better yield was obtained at a higher temperature (about 750). Apossible explanation of this may be that the tighter charcoal packingallowed for more contact time, which favored better reactions at lowertemperatures, whileat higher temperatures this may .have given rise to:more side reactions.

The results of the pyrolyses in Vycor glass tubes show considerablylower yields of hexafluorobenzene, as well as poor recovery oftribromofluoromethane. Increasing temperatures again produce loweryields. ,As one might expect, the interaction of fluorine-containingmaterials with glass results in the removal of fluorine atoms andconsequently poorer yields of fiuoro compounds.

Finally, a number of runs were performed in platinum tubes. The firstthree runs listed in Table I were done under nitrogen pressure of 1atmosphere. The optimum yield obtained 'was 30 percent gross, or 32percent net, at 600. However, a noticeable improvement in the yield wasachieved by performing the pyrolyses under nitrogen pressure of 2-20atmospheres. Under these conditions the efficiency of the reaction wasimproved as is evidenced by a lowered optimum temperature. .The highestyield of hexafluorobenzene was obtained at 540 to 550" with the netyield reaching about 55 percent. I

Table I PYROLYSIS OF TRIBRO MOFLUOROMETHANE Pres- Feed Total BromineGross Recov- Net Tube material Temp., sure, rate, recovliber- Yield,ery, Yield, 0. atm. gJmln. ery, ated, 05F5, CFBI's, CflFfl PercentPercent Percent Percent Percent Graphite I 642-654 1. 3. 90 63 24 18 30D 1. 0 3. 5 96 85 23 8 25 Do 1. 0 4.1 93 91 19 1 19 Do a 96 a a a a 971.0 7.3 95 91 24 8 2e 1. 0 3. 4 88 85 11 1 11 Vycor 1. 0 4. 5 93 3 101.0 5. 1 89 4 1 4 1. 0 8. 3 97 30 14 55 31 1. 0 8.2 98 74 30 8 32 1. 08.9 93 89 18 0 l8 4. 5 0. 5 04 24 12 68 36 4. 5 5. 1 99 50 29 43 50 4. 57. 0 95 62 38 30 55 Platinum 650 4. 5 3. 5 96 67 40 52 580 4. 5 8.8 9688 34 4 35 620 4. 5 7. 8 95 88 31 0 31 540-550 8. 3 l1. 7 98. 8 79. 641. 6 16. 5 49. 5 510-550 8. 3 11.7 97. 8 79. 0 41. 7 18. 3 50. 9550-560 13. 3 l1. 7 95. 8 75. 6 37 10. 8 41. 4 550-550 17. 7 17.0 95. 283. 8 41. 1 8. 7 44. 8 540-550 17. 7 8. 5 95. 0 38. 35 25 50. 7 52. 3Nickel 570-580 17. 7 8. 1 90.0 46.5 23. 6 40. 7 41. 2 590-600 17. 7 9. 290. 5 74. 8 29. 7 10. 7 33. 8

. These four tubes were packed with charcoal. All others listed werepacked with platinum.

Tentative identification of some of the by-products was made bycombining the high boiling fractions from all pyrolyses performed ingraphite tubes and refractionating' them under reduced pressure. Samplesof individual cuts were analyzed by a mass spectrometer. A number ofcompounds were tentatively identified. Some of these compounds, in orderof their decreasing abundance, are listed below:

It appears that most of the by-products were bromofluoro derivatives ofethane and propane as well as benzene and toluene. The presence of thelatter seems to indicate that occasionally fluorine was removed insteadof bromine.

In the light of the foregoing evidence, the mechanism of the reaction ispostulated, which consists of the following four steps:

Steps 1 and 2 should proceed rapidly at the temperature of the reaction.Increased pressure should have no effect on Step 1 and a relativelyminor efiect' on Step 2. Step 3 would be the rate-determining step whichshould be Strongly pressure dependent. The postulated intermediate C FBr however, would be expected to be very unstable and decompose rapidlywith the loss of bromine. One of the possible products of thisdecomposition should be hexafluorobenzene. The effect of presure wouldbe to drive the reaction to the left but again the rapidity of thebromine removal could be expected to overcome the opposing effect ofpressure.

The residues (B.P. 115) of all the pressure runs in the platinum tubewere combined and subjected to a steam distillation. The volatilefraction (79 percent) was dried and pyrolyzed again under the optimumconditions listed above. The material recovery was essentiallyquantitative. The weight of bromine removed amounted to 34 percent byweight of the recovered products; A partial analysis of the remainingfraction. was as follows: 4.6, percent byweight hexafiuorobenzene. 15.5percent bromopentafluorobenzene, and small amounts of some c F Brproduct.

From these results it is quite evident, then, that the serted betweenthe. traps.

residues obtained from the initial pyrolysis of CFBr still containintermediates, suchas CFBr=CFBr and CFBm-CFBr which are capable ofproducing more hexafluorobcnzene by further concentration and pyrolysis.This, however, improves the net yield of'C F by only a few percent.

Experimental procedure T ribr0m0flu0r0methane.-This compound wasprepared essentially by using stoichiometric amounts oftetrabromomethane and antimony trifluoride in the pres- 'cnce of a smallamount of bromine. Standard glass equipment was used. By refluxing for4-6 hours from a bath kept at to and subsequent distillation, yields of60 to 70 percent were obtained. The product was purified by washing withsodium bisulfite and sodium hydroxide. After drying over calciumchloride the product was fractionated in a four-foot column.

- EXAMPLE I H exaflubrabenzene.--Pyrolysis of tribromofluoromethanewas'performed in a platinum tube 80 cm. long, 1 cm. wide and about 1 mm.thick. An electrically heated furnace was wound around this tube and-thetemperature of the furnace was controlled by an automatic relay. Theentrance and exit of the tube were. cooled by watercooled brasscondensers. Tribromofluoromethane was introduced from a reservoir bymeans of a solenoid pump. The outlet of the pyrolysis tube was connectedto an icecooled trap followed by Dry Ice traps. For the runs performedat higher pressures a capillary tube was in This proved to be adequatefor keeping the system under pressure and ensuring a slow and even flowrate. In a typical run tribromofluoromethane was introduced into thereservoir, and a slow stream of dry; nitrogen was passed through thesystem while the furnace was brought to the desired temperature- Therelay controlling the addition of tribromofluoromethane was thenactivated and the reagent added at a constant rate. After completion ofthe addition the product in the receiver and traps was carefully'washedwith sodium bisulfite and dilute sodium hydroxide. The loss of brominewas determined by weighingthe product before and after. washing. After:drying, the washed product was fractionated. tov recoverhexafluorobenzene (13.1. 79 to 81) and unreacted tribromofluoromethane(B.P. 106 to. 108.. The. residues were saved for further purificadon, V

nxmm 1'1 The experiments in graphite and Vycor tubes were performed insimilar apparatus. The tubes were inserted vertically into a ceramicfurnace with the inlet and outlet cooled by brass condensers. Thedimensions of the tubes were 86 cm. long, 1.3 cm. wide and 2 mm. thick.The

heated length of the tubes was 60 cm. The graphite tube was insertedinto a tight fitting iron tube to ensure even heating and to protect thetube from breaking. At the top of the tube a dropping funnel with abypass tube was inserted by means of a rubber stopper. The outlet of thepyrolysis tube was connected to two traps, one cooled in ice, the otherin Dry Ice. Again dry nitrogen was passed through the system during therun. The rate of addition was controlled by setting the stopcock of thedropping funnel. The product was worked up as described above, with theadditional step of combining all residues and fractionating them underreduced pressure to obtain samples for mass spectrometer analysis.

It is to be understood that the invention is not limited to the specificexamples herein,-except as defined in the appended claims.

What is claimed is:

1. The method of preparing hexafluorobenzene comprising pyrolyzingtribromofluoromethane in a reaction chamber at a temperature range offrom 530 C. to 550 C. under flowing nitrogengas at pressures from 4.5 to17.7 atmospheres for a time suflicient to convert from 60 to 90 percentof the tribromofluoromethane.

2. The invention of claim 1 in which the pyrolyzation is conducted in aplatinum reaction chamber at a tem perature range of 530 to 550 C. andin which the pressure of said flowing nitrogen lies within the range offrom 4.5 to 10 atmospheres.

3. The invention of claim 2 in which said pyrolyzation is conducted at atemperature of about 540 C. and at a pressure of about 4.5 atmospheres.

4. The invention of claim 2 in which said pyrolyzation is conducted in atemperature range of from 540 to 550 C. and at a pressure of about 10atmospheres.

5. The invention of claim 1 in which the pyrolyzation is conducted in anickel reaction chamber at a'temperature range of from 540 to 550 C. andthe pressure of said flowing nitrogen is about 17.7 atmospheres.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Desirant: Bull. Classe Sci. Acad. Roy. Belg, vol. 41,

1. THE METHOD OF PREPARING HEXAFLUOROBENZENE COMPRISING PYROLYZINGTRIBROMOFLUOROMETHANE IN A REACTION CHAMBER AT A TEMPERATURE RANGE OFFROM 530*C. TO 550* C. UNDER FLOWING NITROGEN GAS AT PRESSURES FROM 4.5TO 17.7 ATMOSPHERES FOR A TIME SUFFICIENT TO CONVERT FROM 60 TO 90PERCENT OF THE TRIBROMOFLUOROMETHANE.